The Interpolation feature allows you to estimate between your data points. If you have a linear regression or curve fit fitted to your data, then the interpolation will reflect values from the fitted line at the current cursor position, otherwise, the interpolation will reflect values from a straight line drawn between the two points closest to the cursor.
Choose Interpolate from the Analyze menu to turn on the interpolation feature. A helper object will be displayed. Double-click the helper object to display the Interpolate Options dialog. Uncheck the box next to Show on Graph to hide the helper object from the graph. Choose Additional Object Options > Reveal Hidden Objects from the Options menu to return it to the graph.